Abstract: The determinants of aging are complex and multivariate. Genetic studies in the nematode Caenorhabditis elegans and in mice have identified conserved genes and pathways that regulate lifespan. Indeed, drugs targeting these pathways are under development. It has become increasingly evident that while significant extension of lifespan might ostensibly seem desirable, spending the last years of one's prolonged existence gradually becoming more infirm might not actually be all that appealing. The real need rather is to extend `healthspan', that is, the length of time one remains healthy and free of age-related infirmities. Extending healthspan would significantly reduce the burden of geriatric disease, which at present accounts for 23% of the total global burden of disease in people aged 60 years and older and will only increase over the next 20 years (1). The commensal microbiota are becoming increasingly recognized as a determinant of disease severity; therefore, the question arises as to whether the microbiota can regulate healthspan. Our central hypothesis is that the microbiota communicate with pathogens, with the immune system and with tolerance and repair mechanisms by means of soluble secreted small molecules. These molecules orchestrate disease susceptibility and severity, factors critical to healthspan and frailty. Using C. elegans as a ?biosensor,? we identified indole and several metabolic derivatives as factors secreted by the commensal microbiota that may serve this function. We found that brief exposure to indoles activates a signaling pathway in C. elegans in which animals become resistant to a variety of stressors, a process we termed ?conditioning(4).? Conditioning pathways activated by indoles appear to protect the integrity of the intestinal epithelial barrier, and are conserved in C. elegans, Drosophila and mammals. Preliminary data suggest these pathways regulate the sensitivity of mammalian intestinal epithelia to damage induced by pathogens, by radiation, or by autoimmune responses (Graft vs. host disease (GvHD)). Our preliminary data also show that indoles extend healthspan of geriatric worms, flies, and mammals. Here, we test the hypothesis that in geriatric mice the microbiota act via indoles to reduce permeability of the intestinal epithelia, to increase tolerance for damage, and to induce barrier regeneration and repair, and so extend healthspan. In Aim 1, we use geriatric mice, knockout mice, and specific inhibitors to test whether pathways identified in C. elegans and mice mediate the capacity of indoles to maintain mucosal barrier integrity and induce repair, and thus increase healthspan. In Aim 2, we test they hypothesis that indoles can overcome ?inflammaging? associated with epithelial barrier breakdown. These experiments have medical relevance and are appropriate for PAR-15-190, as they will facilitate development of orally administered indoles as means to promote and extend healthspan in aged people. %